1. Field of the Invention
This invention relates to semiconductor lasers, and in particular to buried heterostructure lasers.
2. Description of the Related Art
Fiber optic systems offer several advantages over coaxial links for RF transmission, such as low cost, low attenuation, light weight, immunity from electromagnetic interference, large bandwidth, and novel signal processing capabilities. The key building block for these systems is an RF fiber optic link (delay line that consists of a microwave modulated light source, an optical fiber cable, and a high-speed optical detector). Recent advances in fiber optic technology have resulted in single-mode fibers with tens of gigahertz bandwidths, and in the demonstration of high-speed (about 20 GHz bandwidth) GaInAsP/InP detectors that cover the wavelength range from 1.0 to 1.7 micrometers. Low threshold current single-mode 1.3 .mu.m GaInAsP/InP semiconductor lasers have also been demonstrated. The laser source in the RF fiber optic link can be modulated either directly or with an external modulator. External modulators offer, at present, a slightly wider modulation bandwidth (about 19 GHz), and the possibility of being used in conjunction with laser sources that need only to be optimized for noise reduction. Direct modulation is, undoubtedly, a simpler scheme that offers a high optical throughput. Furthermore, it consumes less drive power than external modulators.
Future airborne and space-borne phased array radar systems will probably require RF fiber optic links for signal distribution, power combination, phase shifting, and beam formation. Much work is being done on the development of high-speed and low-noise semiconductor lasers, high-speed photodetectors, wideband electro-optical modulators, and wideband RF fiber optic links. Because of the possibility of transmitting microwave signals through optical fibers, much effort has been expended in extending the modulation bandwidth of semiconductor lasers to 10 GHz and beyond. There exists a need to develop a semiconductor laser with a modulation bandwidth of about 20 GHz. The availability of such a laser would enable an extremely low noise floor to be attained for RF fiber optic links that operate in the X band (8-12 GHz) and below.
The fabrication of high-speed semiconductor lasers was originally concentrated on GaAlAs/GaAs lasers that emit at 0.85 .mu.m. The well-developed material technology of GaAlAs/GaAs enabled devices of sophisticated design to be fabricated. A direct modulation bandwidth of 12 GHz was first reported for a window-type buried heterostructure (BH) GaAlAs/GaAs laser fabricated on semi-insulating substrates. This is described in the paper by K.Y. Lau et al. in Applied Physics Letters, Vol. 45, pages 316-318, August 1984. In this work the device structure made use of a GaAlAs window of a larger bandgap than the active layer to increase the output power limit before catastrophic mirror damage occurred. The semi-insulating substrate also served to reduce chip capacitance. However, the promise of larger repeater spacing at 1.3 .mu.m for telecommunication links (of 10 km or longer) stimulated intense material research on the quaternary material GaInAsP, which can be grown lattice-matched to InP substrates and operated in the wavelength range between 0.95 and 1.7 .mu.m. This led to the demonstration of quaternary single-mode lasers that possess threshold currents comparable to their GaAs counterparts. Furthermore, the stability of their cleaved facets to high photon densities led to the conjecture that quaternary lasers would possess intrinsic modulation bandwidths that would be larger than those of GaAlAs/GaAs lasers.
That conjecture was realized with the development of a class of buried heterostructure lasers widely referred to as constricted mesa BH lasers. The fabrication of these lasers employs a hybrid combination of liquid-phase epitaxy (LPE) for the growth of the double heterostructure, and regrowth procedures that make use of the techniques of mass transport or vapor phase epitaxy to deposit burying layers of a small (about 1 .mu.m) lateral width for optical confinement. The latter technique was first demonstrated with metal-organic chemical vapor deposition (MOCVD) by Ng et al., as reported in the paper published in Applied Physics Letters, Vol. 39, pages 188-189, August 1981. Two high-speed constricted mesa BH lasers are reported in the following papers: C.B. Su et al., Elect. Lett., Vol. 21, pages 577-578, June 1985; and J. E. Bowers et al., Applied Physics Letters, Vol. 47, pages 78-90, July 1985.
The modulation response of a semiconductor laser can be considered as the superposition of an intrinsic modulation response that is governed by photon-carrier interactions in the diode cavity, and a gradual roll-off attributable to chip and packaging parasitics. The upper limit of the intrinsic modulation response is set by the resonance frequency (f.sub.o), at which a pronounced resonance peak is theoretically predicted and observed. Hence, a primary concern in the design of high-frequency semiconductor lasers is to maximize f.sub.o, the square of which is directly proportional to the power output and differential gain of the device, and inversely proportional to the diode cavity volume. Any improvements gained in the output power of the device without expanding its optical mode volume will lead to a corresponding increase of the maximum resonance frequency. In practice, a parasitic "roll-off" often limits the observed modulation response to a 3-dB bandwidth given approximately by the inverse of a time constant determined, in turn, by the chip capacitance (C.sub.j) and series resistance (R.sub.s).
The modulation of related-art 1.3 lm lasers indicates that the intrinsic modulation bandwidth of these devices typically saturates at a "maximum" resonance frequency of about 12-15 GHz. This is due to the saturation of their output power, or equivalently, a decrease of their differential quantum efficiency at high current biases. The loss of differential quantum efficiency in these devices can be attributed to a combination of junction heating and undesirable leakage current increases at high bias currents. This suggests that a further increase of the "maximum" resonance frequency or modulation bandwidth could only be achieved by designing a new device structure that would utilize the injected current more efficiently; i.e., improve the differential quantum efficiency and hence obtain more output power for the same bias current. Such a device would have the best potential for advancing beyond the current state of the art, and meet the goal of attaining a 20 GHz modulation bandwidth at room temperature.
An important aspect of achieving a high differential quantum efficiency, and therefore optical power, in stripe geometry lasers is to channel the injected current as efficiently as possible into the active region. In buried heterostructures this is accomplished by preventing the bias current from flowing across the burying layers that provide transverse mode confinement. FIGS. 1(a) and 1(b) show two previously reported 1.3-um lasers. The InP structure of FIG. 1(a) is formed on an n-type substrate 2. A p-type mesa consisting of a buffer layer 4 and clad layer 6 over the central portion of the buffer layer is formed on the substrate, with an active region 8 between the buffer and clad layer. The clad layer 6 provides carrier confinement to the mesa, and is doped to confine light within the active region. To further confine current flow to the mesa, a p-type layer 10 and n-type layer 12 are grown in succession over the lower portion and adjacent the sides of the mesa, forming an n-p blocking junction. A dielectric layer 16 is formed over the top of clad layer 6 to confine the growth of the blocking layers 10 and 12 to the sides of the mesa, and make sure none gets on top of the mesa. The laser structure of FIG. 1(b) is similar, but the doping of the substrate 2', buffer layer 4', clad layer 6' and blocking layers 10' and 12' are reversed from their counterparts in FIG. 1(a).
The fabrication of complementary laser structures on p-InP substrates was reported by Y. Nakano et al. in a paper in Elect. Lett., Vol. 17, pages 782-783, October 1981. A high-power GaInAsP/InP etched mesa buried heterostructure laser fabricated on a p-substrate was reported by Nakano et al. The difference in the output power obtained between the complementary structures FIGS. 1(a) and 1(b) is illustrated in FIG. 2. The experimental results demonstrated that a p-n blocking junction (as in a p-substrate BH laser) is superior to a n-p blocking junction (as in an n-substrate BH laser). The underlying physical mechanism responsible for the difference was explained by the presence of an abnormally high resistivity region near the surface of p-type InP. The investigation reported by Y. Nakano et al. in Japan J. Appl. Phys., Vol. 20, No. 8, L619-L622, August 1981, shows that a blocking junction formed by growing a p-InP layer on top of an n-InP layer has a higher, more stable breakdown voltage than its complementary counterpart. It is, therefore, more efficient in confining current flow through the active region.
While the p-substrate device attains a higher output power, both devices suffer from a leakage current along the lateral faces of their respective clad layers that bypass the blocking junction, and can create a short to the substrate. These leakage currents have been traced to crystal face defects along the sides of the clad layers, as reported by R. A. Logan, et al. in J. Appl. Phys., Vol. 54(9), p. 5462-5463, Sept. 1983.